Light emitting device and method for enhancing light extraction thereof

ABSTRACT

A method for enhancing light extraction of a light emitting device is disclosed. The method includes the steps of: providing a site layer on the light emitting device; placing a protection layer on the site layer; forming an array of pores through the protection layer and the site layer; and growing on the site layer an oxide layer, having a plurality of rods, each of which is formed in one of the pores. The shapes of the rods can be well controlled by adjusting reactive temperature, time and N 2 /H 2  concentration ratio of atmosphere such that the shape and light escape angle of the rods can be changed.

FIELD OF THE INVENTION

The present invention relates to a method for enhancing light extractionof a light emitting device. More particularly, the present inventionrelates to a method for enhancing light extraction of a light emittingdevice by forming an oxide layer, such as a zinc oxide layer, with acontrollable roughness on the light emitting device.

BACKGROUND OF THE INVENTION

Recently, since development of integrated circuits (IC) has been downsized to nano-scale, application to nano-scale elements becomes more andmore popular. Among all inventions, short-wavelength light emittingdevices, such as laser diodes (LD) and light emitting diodes (LED), havebeen the mainstream. For development of short-wavelength light emittingdevices, III-V compounds semiconductors are the common materials formanufacturing LED. However, with development of new systematicmaterials, II-VI compounds semiconductors are valued again. In practice,Zinc oxide (ZnO) has advantages of low cost and easy synthesis. Hence,study on ZnO is a hot topic today, especially on the ZnO nanorods.

ZnO has a direct band-gap of 3.37 eV which is higher than other highdirect band-gap semiconductor materials. In addition, ZnO has higherexcitation binding energy (excitation binding energy of Gallium Nitride(GaN) is around 20 meV while that of ZnO is much higher and about 60meV.). Therefore, its lighting efficiency is higher than other materialsunder room temperature. During recent years, a lot of reports on studyof ZnO show that it can be applied to short-wavelength elements andlaser diodes due to the good lighting efficiency. Since data accessingcan be improved by using ultraviolet (UV) laser, application of ZnO toUV laser source has a great potential. For ZnO, membrane elements arevery popular.

Besides, another main direction of development of ZnO is one-dimensionalnanorods (nanowires). Scientists can grow highly aligned nanorod arraysuccessfully. With photoluminescence, UV laser is excited out of thenanorods. Although UV laser can be commercialized in many ways, how toenhance light extraction and control light escape angle of nanorodsstill has two problems to be solved. Otherwise, lighting efficiency willbe significantly affected.

Therefore, the present invention provides a solution to the problemsmentioned above. Via the invention, light extraction of a light emittingdevice can be enhanced.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide amethod for enhancing light extraction of a light emitting device byforming an oxide layer with a controllable roughness on the lightemitting device.

In accordance with an aspect of the present invention, a method forenhancing light extraction of a light emitting device, includes thesteps of: providing a site layer on the light emitting device; placing aprotection layer on the site layer; forming an array of pores throughthe protection layer and the site layer; and growing on the site layeran oxide layer, having a plurality of rods, each of which is formed inone of the pores. The shapes of the rods can be well controlled byadjusting N₂/H₂ concentration ratio, reactive temperature and time suchthat the shape and light escape angle of the rods can be changed.

Preferably, the oxide layer comprises zinc oxide (ZnO), silicon dioxide(SiO₂), titanium dioxide (TiO₂), or aluminum oxide (Al₂O₃).

Preferably, the oxide layer is formed by hydrothermal treatment, sol-gelmethod, electro-plating, thermal evaporation, chemical vapor deposition(CVD), or molecular beam epitaxy (MBE).

Preferably, the site layer comprises ITO, Ni/Au, NiO/Au, p-ZnO, or ZnO.

Preferably, the protection layer comprises photoresist material ordielectric material.

Preferably, the atmosphere temperature is higher than 200° C.

Preferably, the atmosphere comprises nitrogen, hydrogen, or a mixturethereof.

Preferably, the atmosphere has a nitrogen/hydrogen concentration ratiolarger than 1.

Preferably, the rods have a nanostructure or a microstructure.

Preferably, the rods have a shape of a hexagonal pyramid or a truncatedhexagonal pyramid.

Preferably, the rod has a bottom surface with a diameter ranging from100 nm˜order of micrometers.

Preferably, the pores are formed by wet etching process, dry etchingprocess, photolithography and exposure development process, lasercutting process, or electron beam writing process.

In accordance with another aspect of the present invention, a lightemitting device having enhanced light extraction includes a lightemitting substrate; a site layer provided on the light emittingsubstrate; an array of pores formed in the site layer; a protectionlayer placed on the site layer having the pores exposed; and an oxidelayer formed on the site layer, having a plurality of rods, each ofwhich is formed in one of the pores. The shapes of the rods are adjustedby controlling temperature and concentration of atmosphere such thatlight escape angle of the rods can be changed.

Preferably, the oxide layer comprises zinc oxide (ZnO), silicon dioxide(SiO₂), titanium dioxide (TiO₂), or aluminum oxide (Al₂O₃).

Preferably, the oxide layer is formed by hydrothermal treatment, sol-gelmethod, electro-plating, thermal evaporation, chemical vapor deposition(CVD), or molecular beam epitaxy (MBE).

Preferably, the site layer comprises ITO, Ni/Au, NiO/Au, p-ZnO, or ZnO.

Preferably, the protection layer comprises photoresist material ordielectric material.

Preferably, the atmosphere temperature is higher than 200° C.

Preferably, the atmosphere comprises nitrogen, hydrogen, or a mixturethereof.

Preferably, the atmosphere has a nitrogen/hydrogen concentration ratiolarger than 1.

Preferably, the rods have a nanostructure or a microstructure.

Preferably, the rods have a shape of a hexagonal pyramid or a truncatedhexagonal pyramid.

Preferably, the rod has a bottom surface with a diameter ranging from100 nm˜order of micrometers.

Preferably, the pores are formed by wet etching process, dry etchingprocess, photolithography and exposure development process, lasercutting process, or electron beam writing process.

BRIEF DESCRIPTION OF THE DRAWINGS

The above objects and advantages of the present invention will becomemore readily apparent to those ordinarily skilled in the art afterreviewing the following detailed description and accompanying drawings,in which:

FIG. 1 is a flow chart of a preferred embodiment according to thepresent invention;

FIG. 2 is a three dimensional view of the present invention;

FIG. 3 is a cross-sectional view along the A-A′ cross section in FIG. 2to show formation of pores;

FIG. 4 is cross-sectional view along the A-A′ cross section in FIG. 2 toshow formation of rods;

FIG. 5 is an enlarged view of a rod;

FIG. 6 shows structures of the rods formed under different atmospheres;

FIG. 7 shows a light path in the rod which has a shape of a hexagonalcolumn; and

FIG. 8 shows a light path in the rod which has a shape of a hexagonalpyramid.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention will now be described more specifically withreference to the following embodiment. It is to be noted that thefollowing descriptions of preferred embodiment of this invention arepresented herein for purpose of illustration and description only; it isnot intended to be exhaustive or to be limited to the precise formdisclosed.

FIG. 1 is a flow chart of a preferred embodiment according to thepresent invention showing a method for enhancing light extraction of alight emitting device by forming an oxide layer with a controllableroughness on a light emitting device. FIG. 2 is a three dimensional viewof the present invention. It also shows the relative position for eachelement in the present invention. Now, please refer to FIGS. 1 and 2.The method of the present invention for enhancing light extraction of alight emitting device by forming an oxide layer with a controllableroughness on a light emitting device includes the following steps.First, a light emitting device 102 having a surface layer 104 formed onthe top surface thereof is provided (as shown at step S101). In thisembodiment, the surface layer 104 is made of p-GaN. However, it shouldbe noted that the surface layer 104 of the present invention is notlimited to p-GaN. It can also be made of p-AlGaN, p-InGaN, p-GaN/InGaNSLs, p-AlGaN/GaN SLs, p-AlInGaN, p-InAlGaN/InAlGaN SLs, n+-(In)(Al)GaN,ITO, p-ZnO, ZnO, or Ni/Au. In other words, the surface layer 104 is notlimited to a conductive type of P or N.

The light emitting device 102 used in this embodiment is a nitride lightemitting diode which has an energy band gap equivalent to wavelength of200 to 650 nm.

Later, a site layer 106 is provided on the surface layer 104 of thelight emitting device 102 (as shown at step S102). The site layer 106can be made of ITO, Ni/Au, NiO/Au, p-ZnO, or ZnO.

Next, a protection layer 107 is placed on the site layer 106 (as shownat step S103). Please refer to FIG. 3. It is a cross-sectional viewalong the A-A′ cross section in FIG. 2. An array of pores 109 are formedthrough the protection layer 107 and the site layer 106 by wet etchingprocess, dry etching process, photolithography and exposure developmentprocess, laser cutting process, or electron beam writing process (asshown at step S104). The protection layer 107 is made of photoresistmaterial while photolithographic process is used, whereas the protectionlayer 107 is made of dielectric material while etching process is used.

Then, an oxide layer having a plurality of rods 108 is grown on the sitelayer 106 such that each of the rods 108 is formed in one of the pores109. The protection layer 107 is for positioning the rods 108 into thepores 109 and preventing the rods 108 from growing at a place other thanthe pores 109.

The shapes of the rods 108 are adjusted by controlling temperature andconcentration of atmosphere such that light escape angle of the rods canbe changed (as shown at steps S105˜S106).

The oxide layer can be made of zinc oxide (ZnO), silicon dioxide (SiO₂),titanium dioxide (TiO₂), or aluminum oxide (Al₂O₃). In this embodiment,the oxide layer is made of ZnO.

In this embodiment, the oxide layer is formed by hydrothermal treatment.First, the light emitting device including the site layer is cleanedwith acetone, methanol, and deionized water for about 5 minutes,respectively. Then, the light emitting device and the site layer aredried by a nitrogen spray gun. Next, a seed layer of ZnO is formed onthe site layer for increasing adhesion. The light emitting device, sitelayer, and seed layer is together called a mediator.

The seed layer of ZnO is prepared by dissolving zinc acetate(Zn(CH₃COO)₂.H₂O) in 2MOE (CH₃O(CH₂)₂OH, 2-methoxyethanol), each havinga concentration of 0.5M, and then stirring the resultant solution for 2hours while heating at a temperature of 65° C., so that a transparentgel solution is obtained. Later, the transparent gel solution is spincoated onto the top surface of the site layer. Next, a ZnO seed layer isobtained by thermal annealing having the transparent gel solutiondeposited thereon at a temperature of 130° C. for 60 minutes. In thisembodiment, the ZnO seed layer is used for ZnO particles to grow as aZnO layer.

It should be understood that the seed layer is not limited to be made ofZnO, and can also be made of gold (Au), silver (Ag), Tin (Sn), or cobalt(Co). The oxide layer may be formed randomly or orderly.

After the seed layer is formed, the mediator is placed facing downwardsin a growth solution of zinc nitrate hexahydrate (Zn(NO₃)₂.6H₂O) havinga purity of 98% and hexamethylenetetramine (C₆H₁₂N₄, HMT) having apurity of 99.5%, each having a concentration of 0.5M. Later, it isheated in a dryer at a low temperature of 90° C. for about 3 hours.After being heated, it is taken out and washed with deionized water.Then, a ZnO layer having a plurality of rods could be obtained.Otherwise, the growth rate, dimension and height of ZnO rods can be wellcontrolled by adjusting the temperature, concentration and growth time.

During the hydrothermal treatment, ZnO is formed according to thefollowing formulas:

In the aforementioned deposition mechanism, ZnO begins to form onto theseed layer once the concentrations of zinc ions and hydroxide ions reachsaturation. Due to anisotropic characteristic of atomic bonding, atomstend to flow towards low energy during nucleation causing asymmetricgrowth in a specific direction which thereby forms a rod/thread shapearray structure.

Although hydrothermal treatment is used in the present embodiment, itshould be understood that the present invention is not limited tohydrothermal treatment, and can also use thermal evaporation, sol-gelmethod, chemical vapor deposition (CVD), or molecular beam epitaxy(MBE).

Moreover, even though spin coating is used for disposing the seed layeronto the GaN substrate in the present embodiment, it should not belimited thereto, and can also use dip coating, evaporation, sputtering,atomic layer deposition, electrochemical deposition, pulse laserdeposition, metal-organic chemical vapor deposition, or thermalannealing.

The atmosphere can include nitrogen, hydrogen, or a mixture thereof. Inthis embodiment, the atmosphere has a nitrogen/hydrogen concentrationratio larger than 1 while the atmosphere includes a mixture of nitrogenand hydrogen. Moreover, the atmosphere temperature for the controllableroughness of ZnO rods is higher than 200° C.

Under the aforementioned atmosphere conditions, the rods 108 may haveshapes of hexagonal pyramids or truncated hexagonal pyramids with a sizeorder between nanometer and micrometer. The rods 108 have a bottomsurface with a diameter ranging from 100 nm˜order of micrometers.

Please see FIG. 4. The sketch illustrates how the rods 108 of the ZnOlayer are formed in the pores 109 of the site layer 106. Each rod 108has a hexagonal cross section and a Wurtzite structure as shown in FIG.5. In the present invention, it has a hexagonal column appearance. Thelight beams emit in a specified direction from the light emitting device102 according to the structure of the rods 108.

Please refer to FIG. 6. FIG. 6 illustrates different shapes of the rod108 which are formed under different temperature and concentration ofatmosphere conditions. For example, the rod 108 shown in FIG. 6A whichis shaped as a hexagonal pyramid is formed under an atmosphere having anitrogen/hydrogen concentration ratio smaller than that of FIG. 6C whichis shaped as a truncated hexagonal pyramid. Furthermore, the rod 108shown in FIG. 6A can also be achieved by providing a temperature higherthan that of FIG. 6C. Therefore, roughness of a light emitting devicecan be controlled by changing the shape (sharpness) of the pyramids ofthe rods 108 of the oxide layer.

Please refer to FIGS. 7 and 8. These two figures show different lightpaths in a hexagonal prism and a hexagonal pyramid. It is obvious thatthe hexagonal pyramid will change light direction by refraction andincrease the light escape angle, which can be compared with FIG. 7.Therefore, the light extraction of the light emitting device can beenhanced and the light escape angle can be controlled by the presentinvention.

While the invention has been described in terms of what is presentlyconsidered to be the most practical and preferred embodiment, it is tobe understood that the invention needs not be limited to the disclosedembodiment. On the contrary, it is intended to cover variousmodifications and similar arrangements included within the spirit andscope of the appended claims, which are to be accorded with the broadestinterpretation so as to encompass all such modifications and similarstructures.

1. A method for enhancing light extraction of a light emitting device,comprising the steps of: providing a site layer on the light emittingdevice; placing a protection layer on the site layer; forming an arrayof pores through the protection layer and the site layer; and growing aplurality of oxide rods on the light emitting substrate, each of whichis formed in one of the pores; wherein the site layer and the protectionlayer are formed of different materials, and wherein the shapes of therods are controlled by adjusting reactive temperature, time and N₂/H₂concentration ratio of atmosphere such that the shape and light escapeangle of the rods can be are changed.
 2. The method according to claim1, wherein the oxide rods comprise zinc oxide (ZnO), silicon dioxide(SiO₂), titanium dioxide (TiO₂), or aluminum oxide (Al₂O₃).
 3. Themethod according to claim 1, wherein the oxide rods are formed byhydrothermal treatment, sol-gel method, electro-plating, thermalevaporation, chemical vapor deposition (CVD), or molecular beam epitaxy(MBE).
 4. The method according to claim 1, wherein the site layercomprises ITO, Ni/Au, NiO/Au, p-ZnO, or ZnO.
 5. The method according toclaim 1, wherein the protection layer comprises photoresist material ordielectric material.
 6. The method according to claim 1, wherein theatmosphere temperature is higher than 200°.
 7. The method according toclaim 1, wherein the atmosphere comprises nitrogen, hydrogen, or amixture thereof.
 8. The method according to claim 1, wherein theatmosphere has a nitrogen/hydrogen concentration ratio larger than
 1. 9.The method according to claim 1, wherein the rods have a nanostructureor a microstructure.
 10. The method according to claim 1, wherein therods have a shape of a hexagonal pyramid or a truncated hexagonalpyramid.
 11. The method according to claim 1, wherein the rod has abottom surface with a diameter ranging from 100 nm˜1 μm.
 12. The methodaccording to claim 1, wherein the pores are formed by wet etchingprocess, dry etching process, photolithography and exposure developmentprocess, laser cutting process, or electron beam writing process.13.-24. (canceled)